Foods high in complex carbohydrates — such as fruits, vegetables, and whole-grain bread, rice, or pasta — are excellent pre-exercise snacks. The complex carbohydrates enter your bloodstream slowly and help you avoid sugar crashes, which can occur when you eat snacks that are made with simple sugars, such as candy. Many energy bars are loaded with complex carbs, but I find the energy bars to be much too filling for a pre-exercise snack. Here's what I do: I cut an energy bar in half so I can eat one piece before my workout, the other piece the next day.
Exactly how much you should eat before working out is really a personal thing. Some people can consume a large meal just before exercising and not experience any problems, while others can barely eat half an apple without getting sick during their workout. This is really something you need to experiment with to find out what, when, and how much of a snack works best for you.